This invention relates to electrochemical cells and batteries, especially lead-acid, and particularly to a welded connection between current collector tabs of the plates and battery straps attached to the customary inter cell connectors as well as the posts connected to the output terminals of the cell.
Current collector tabs attached to common polarity electrode plates are customarily joined together by a connecting strap (i.e., a buss bar) connected or integral at one end with a post serving as a terminal or inter cell connector for a battery. The current collector tabs emanating from the plates have traditionally been joined to the battery straps by a burning process with the aid of lead filler rods. Certain high performance batteries requiring a minimum impedance connection may use more sophisticated methods such as disclosed in U.S. Pat. No. 3,806,696 to Young et al. and U.S. Pat. No. 4,037,077 to Harder. In those patents, a leg portion of the strap is sandwiched between collector tabs and surrounded by an ingot-like mold, provided for instance by tungsten jaws, and then an electrode arc derived from an inert-gas-shielded, non-consumable electrode (e.g., TIG or plasma) is oscillated over the lead parts causing them to melt and fuse together. The high thermal conductivity of the mold jaws act as a heat sink serving to quickly cool and form the weld bead.
Applicant has discovered, especially when employing the method of the Young or Harder patents, that an unacceptably high percentage of the collector tab-to-strap welds possess latent defects. A loss of electrical connection occurs upon subsequent use and/or vibration of the battery due to a condition in which only the collector tabs have been fused together; the fused tabs bridge over the leg of the connecting strap without melting together with the leg as desired. Initial testing shows a good connection solely because of frictional contact between the leg of the strap and tabs. These "bridge welds" increase the internal impedance of the cell, and oftentimes lead to a loss of connection between the output terminals and plates of the battery, producing an open circuit.
It is a primary object of the invention to minimize the chance of forming bridge welds in a current collector tab-to-strap connection in a lead or other electrochemical cell or battery, and to provide a reliable, cheap unitary welded low silhouette connection without appreciably increasing the weight or mass of the strap. It is another object, especially with respect to the manufacture of automotive (SLI) lead batteries to provide a lead strap design eliminating the need for lead filler rods.